The invention relates to the field of ionizers for gas cluster ion beam formation.
It has been shown that ionized beams of gas clusters can be accelerated and directed toward a target or work-piece surface to produce various desirable effectsxe2x80x94cleaning, smoothing, sputter etching, deposition, implantation, etc. A body of literature describes the creation and application of gas cluster ion beams. See, for example, U.S. Pat. No. 5,814,194 issued to Deguchi et al.
FIG. 1 is a functional block diagram of a conventional gas cluster ion beam processing system 100 with a differential vacuum pumping scheme. Gas clusters are formed by creating, with a properly shaped nozzle 102, a cluster jet of gas flowing into a region 104 of substantially reduced pressure. Cooling by adiabatic expansion causes the gas to condense into clusters of from several to several thousand atoms or molecules. A gas skimmer aperture 106 is used to separate the gas molecules that have not been converted into clusters from the cluster jet, minimizing pressure in the downstream regions where such higher pressures would be detrimental (e.g., ionizer 108, accelerator 110, and process chamber 112).
Although not essential, it is sometimes desirable to also employ a differential vacuum pumping scheme, as shown in FIG. 1, to further help isolate the downstream regions from the higher pressure source region. Suitable source 114 gases are (for example) argon, other inert gases, oxygen, nitrogen, oxygen bearing gases such as carbon dioxide, nitrogen bearing gases, halogens and halogen bearing gases. Because, during cluster impact, transiently high temperatures and pressures obtain, chemical reactions are facilitated. Therefore, chemically reactive gases like oxygen, nitrogen, halogens or gases bearing those elements as a constituent are also used because of the surface chemical reactions they can effect. Inert gases process the substrate surfaces by mechanical action. Of course, mixtures of inert gases with reactive gases are also possible.
After the cluster jet (which is a substantially neutral beam of clusters) has been formed, the clusters are ionized in the ionizer 108. The ionizer is typically an electron impact ionizer that produces thermal electrons from one or more incandescent filaments. It accelerates and directs the electrons, causing them to collide with the gas clusters in the gas cluster jet. The electron impact ejects electrons from the clusters, causing part of the clusters to become positively ionized.
Once ionized, suitably biased electrodes extract the cluster ions from the ionizer, focus them to form a beam, and accelerate them to a desired energy, typically from 1 keV to several tens of keV or perhaps even a few hundred keV. Not shown, but sometimes utilized, is a mass selector for selecting clusters of a certain mass or within a certain range of masses. Such mass selectors can be, for example, a transverse magnetic field for deflecting monomser ions and other light ions (such as cluster ions having approximately ten or fewer atoms or molecules) out of the beam and passing more massive cluster ions. The accelerated ion clusters may be scanned by scanning mechanisms 116 using well known electrostatic scanning techniques to uniformly process the surface of the target 118 or work-piece. The extracting, focusing, accelerating, mass selecting, and scanning all are made possible by virtue of the fact that the beam of clusters is ionized.
GCIB can be used for many surface processes: smoothing, etching, reacting, depositing, etc. Many of the preferred GCIB processes produce their desired results in proportion to the intensity of the ionized cluster beam that is delivered to the target or work-piece. The beam intensity is dependent on several factors, including: intensity of the neutral gas cluster jet formed by the nozzle (cluster formation efficiency); fraction of the neutral cluster jet that is subsequently ionized (the ionization efficiency); and fraction of the ionized clusters that are subsequently transported to the target or work-piece (the transport efficiency).
Other factors such of the acceleration potential, the cluster size, the constituent gas of the clusters, etc. are also important to the effectiveness and efficiency of the process.
One of the objects of the invention is to improve the efficiency of GCIB processing by increasing the fraction of the neutral clusters in the jet which become ionized (increasing ionization efficiency). Since increased ionization efficiency can increase the GCIB intensity, then it follows that the productivity of various GCIB processes will be improved by increased ionization efficiency.
Accordingly, the invention provides a neutral beam ionizing apparatus for electron impact ionization of a substantially cylindrical neutral beam. The apparatus includes an electron source, and a circularly cylindrical ionizing region that is substantially free of magnetic fields. In one embodiment of the invention, the beam is a gas cluster beam, and the electron source includes a heated filament for emitting thermions, the filament including one or more direction reversals shaped to produce self-nulling magnetic fields so as to minimize the magnetic field due to filament heating current. The filament can be bifilarly, multi-filarly, or non-inductively wound around the circumference of the cylinder. In another embodiment, the cylindrical ionizing region includes a pair of concentric cylindrical electrodes biased so as to cause electrons emitted from the electron source to orbit repeatedly through the axis of the beam to be ionized.
In accordance with another embodiment of the invention there is provided a neutral beam ionizing apparatus for electron impact ionization of a substantially cylindrical neutral beam, the ionizing apparatus including at least one electron source, and an elliptically cylindrical ionizing region. In one embodiment, the elliptically cylindrical ionizing region includes a pair of co-focal elliptically cylindrical electrodes biased so as to cause electrons emitted from the at least one electron source to orbit repeatedly through the axis of the beam to be ionized. The neutral beam axis can lie substantially along a first focus of the elliptical cylinder and an electron source can lie along a second focus of the elliptical cylinder. The at least one electron source can include a non-inductively formed filament surrounded by a cylindrical anode concentric with the second focus of the elliptical cylinder. In an alternative embodiment, the neutral beam axis lies substantially along a first focus of the elliptical cylinder, and the electron sources are disposed to lie outside of the elliptical cylinder and to direct one or more beams of electrons through a focus of the elliptical cylinder. In other embodiments, at least one electron source includes at least one electron gun disposed to direct at least one beam of electrons through the second focus of the elliptical cylinder. In another embodiment, the at least one electron source includes a virtual source such that said electrons travel in orbits appearing to originate at the second focus. In all of the embodiments, the beam to be ionized can be a gas cluster beam.